The action of 3 alkylating carcinogens (dimethylnitrosamine, diethylnitrosamine, and methylnitrosourea) on nucleolar and nuclear DNA in normal and regenerating rat liver will be studied. DNA will be isolated at various time points after treatment. Base alkylations and phosphate alkylations will be quantitatively analyzed by enzymatic digestion to deoxyribonucleoside 3'-monophosphates, enzymatic labeling with a radioactive reagent of high specific activity, separation of the radioactive products by thin layer chromatography, and counting. The main objective of these studies is to determine whether the alkylation of nucleolar DNA differs from that of nuclear DNA and whether replicating DNA in regenerating liver exhibits an altered susceptibility to alkylating agents compared to DNA of normal adult liver. Methods are being developed that will not require the use of radioactive carcinogens to detect minute amounts of adducts present in DNA after exposure to chemical carcinogens.